Malicious software, known as malware, may be defined broadly as any computer data file or data stream that, when directed at a computer system, acts contrary to the interest of the system owner or operator. Malware may be categorized as any of a number of subclasses including computer viruses, worms, trojan horses, rootkits, spyware, etc. The term computer virus is often used synonymously with malware even when used to describe malware that is not generally classified as a virus. Malware detection software may be able to remove malware that is already present on a computer system. However, malware detection software may not provide a physical barrier for blocking malware from reaching the computer system.